1. Field of the Invention
This invention relates to an aqueous stripping composition that is particularly useful for cleaning organic and inorganic compounds or polymers from a substrate. More particularly, the invention describes an aqueous stripping composition having an alkanolamine and tetraalkylammonium hydroxide. Preferably, the invention further includes a metal corrosion inhibitor, and the composition is essentially free of any hydroxylamine compounds.
2. Description of the Prior Art
An industrial process that produces an article of manufacture often repetitively uses one or more elements of the process for economic and reproduciability reasons. In particular, it is quite common for articles to be manufactured on a substrate and then subsequently removed from said substrate. In such a situation, it would be advantageous to be able to reuse the substrate for the manufacture of the next article.
The substrate, however, is often contaminated from the process and needs to be efficiently cleaned without impairing its integrity. Accordingly, there exists a need to develop an improved stripping composition to efficiently clean a variety of substrates from the deposits of a wide variety of compositions. This composition should also be economical, environmental friendly and easy to use.
The present invention teaches such a new and improved stripping composition. This composition is aqueous, dissolves both organic and inorganic substances, and cleans a variety of substrates, thus allowing for the reuse of the substrate for another process.
During manufacture of semiconductors and semiconductor microcircuits, it is frequently necessary to coat the substrates from which the semiconductors and microcircuits are manufactured with a polymeric organic filing, generally referred to as a photoresist, e.g., a substance which forms an etch resist upon exposure to light. These photoresists are used to protect selected areas of the surface of the substrate, while the etchant selectively attacks the unprotected area of the substrate. The substrate is a typically silicon dioxide coated silicon wafer and may also contain metallic microcircuitry, such as aluminum or aluminum alloys on the surface. Following completion of the etching operation and washing away of the residual etchant, it is necessary that the resist be removed from the protective surface to permit essential finishing operations. It is desirable to develop an improved stripping composition to remove the organic polymeric substance from a coated inorganic substrate without corroding, dissolving or dulling the metal circuitry or chemically altering the wafer substrate.
Stripping compositions used for removing coatings from photoresists and other substrates have for the most part been highly flammable, generally hazardous to both humans and the environment, and comprise reactive solvent mixtures exhibiting an undesirable degree of toxicity. Moreover, these stripping compositions are not only toxic, but their disposal is costly since they must be disposed of as a hazardous waste. In addition, these compositions generally have severely limited bath life and, for the most part, are not recyclable or reusable.
Generally, compositions containing chlorinated hydrocarbons and/or phenolic compounds or other highly caustic and corrosive materials have been employed as stripping compositions for stripping paints, varnishes, lacquers, enamels, photoresists, powder coatings and the like, from substrates such as wood, metal or silicon wafers. Hot caustic compositions are generally employed to remove coatings from metals and methylene chloride compositions to remove coatings from wood. In many cases, the components of the stripping compositions are relatively toxic reactive solvent mixtures and thus must be subject to stringent use conditions and require hazardous chemical handling procedures and wearing of safety garments and apparel by users so as to avoid contact with the stripping compositions. Additionally, because many of the toxic components of such stripping compositions are highly volatile and subject to high evaporation rates, they require special human and environmental safety precautions to be taken during storage and use of said compositions.
U.S. Pat. No. 4,276,186 to Bakos et al discloses a non-aqueous cleaning composition which includes N-methyl-2-pyrrolidone and an alkanolamine. However, in a comparative study, applicant has found that the use of N-methyl-2-pyrrolidone does not provide a broad spectrum of cleaning as is capable with the composition of the present invention.
U.S. Pat. No. 4,617,251 to Sizensky discloses a non-aqueous stripping composition which is prepared with a select amine and an organic polar solvent. The composition is formed utilizing from about 2 to about 98% by weight of an amine compound and about 98 to about 2% of an organic polar solvent.
U.S. Pat. No. 4,770,713 to Ward discloses a stripping composition comprising an alkylamide and the alkanolamines which are useful in the present invention.
U.S. patent application Ser. No. 08/162,429 to Ward discloses a non-aqueous stripping composition comprising hydroxylamine, an alkanolamine and water. U.S. Pat. No. 5,279,771 to Lee describes a non-aqueous stripping composition containing hydroxylamine, an alkanolamine and an organic polar solvent. Another Lee reference, U.S. Pat. No. 5,334,332 describes a cleaning solution for resist-free substrates that contains hydroxylamine, an alkanolamine and water. None of these references disclose a hydroxylamine-free composition. Nor do these references suggest, alone or in combination with other prior art, eliminating hydroxylamine from a suitable stripping composition.
The present invention, in contrast, provides an effective stripping composition that comprises an aqueous solution of an alkanolamine a corrosion inhibitor and a tetraalkylammonium hydroxide, wherein the said composition is essentially free of any hydroxylamine compounds. In a copending patent application to Ward, filed herewith, Ward describes an aqueous stripping composition essentially comprising monoethanolamine. The use of a tetraalkylammonium hydroxide in the present application allows for the use of a wider range of chemicals in a wider range of proportions. In particular, the present invention may contain one or more alkanolamines, rather than just monoethanolamine. Moreover, the tetraalklkyl ammononium hydroxide compound works synergistically with the inhibitor to provide superior anti-corrosive properties.
Recently, OSHA, EPA and other similar federal, state and local governmental regulatory agencies have advocated a shift toward using more human and environmentally compatible stripping compositions and stripping methods that are not subject to the aforementioned drawbacks and problems.
Moreover, heretofore available photoresist stripping compositions have required unduly long residence times or repeated applications in order to remove certain coatings. In addition, various coatings have resisted removal from certain substrates with these conventional stripping compositions. That is, the previously available stripping compositions have not provided adequate or complete removal of certain hard-to-remove coatings from various substrates.
It is, therefore, highly desirable to provide stripping compositions that exhibit substantially little human or environmental toxicity, are water miscible and are biodegradable. It is also desirable to provide stripping compositions that are substantially non-flammable, non-corrosive, evidence relatively little, if any, tendency to evaporate, and are generally unreactive and of little toxicity to humans as well as being environmentally compatible.
In addition, it would be desirable to provide photoresist stripping compositions that have a high degree of stripping efficacy, particularly at lower temperatures than generally required with prior stripping compositions.
It is also highly desirable that photoresist stripping compositions be provided that exhibit substantially no corrosive effects on the substrate.
It is also desirable that effective stripping compositions be provided that are devoid of objectionable chlorinated or phenol components and do not require the use of hot caustic components. Most desirable are stripping compositions and use thereof that are not considered undesirable by regulatory agencies overseeing their production and use.
It is also most advantageous that stripping compositions be provided with the above-identified desirable characteristics which evidence synergistic stripping action in that the mixtures of components provide stripping efficacy and stripping results not always obtainable with the individual components.
U.S. Pat. No. 3,972,839 to Murphy discloses a composition useful for stripping paint films comprising one or more alkaline materials and an accelerator. The accelerator comprises a mixture of two different amine compounds. Of significance in Murphy is the disclosure that various alkanolamines, in and of themselves, were ineffective in stripping capabilities. See U.S. Pat. No. 3,972,839 at Table I, col. 4 (e.g., monoethanolamine had no stripping effect after 30 minutes). This is why it is surprising that an alkanolamine in combination with tetraalkylammonium hydroxide is such an effective stripping composition in the present invention.